The CO.sub.2 /O.sub.2 leaching system has already been used commercially for in situ leaching at sites in South Texas. The chemistry of this system is described in detail in the literature. In essence a CO.sub.2 /CO.sub.2 -containing leaching solution, or lixiviant, is pumped through the formation to solubilize insoluble tetravalent uranium in the formation to soluble hexavalent uranium and to remove this dissolved uranium in the pregnant lixiviant from the formation through the production wells. The earlier processes used chlorate ions or hydrogen peroxide to oxide the uranium in the formation:
3UO.sub.2 (S)+ClO.sub.3.sup.- +3H.sub.2 O.fwdarw.3UO.sub.2.sup.+2 (S)+Cl.sup.- +6OH.sup.- UO.sub.2 (S)+H.sub.2 O.sub.2 .fwdarw.UO.sub.2.sup.+2 (S)+2OH.sup.- PA1 UO.sub.2 (S)+[O]+H.sub.2 O.fwdarw.UO.sub.2.sup.+2 (S)+2OH.sup.- PA1 UO.sub.2.sup.+2 (S)+3CO.sub.3.sup.= .fwdarw.UO.sub.2 (CO.sub.3).sub.3.sup.-4 3HCO.sub.3.sup.- .fwdarw.3CO.sub.3.sup.= +3H.sup.+ PA1 UO.sub.2 (S)+[O]+3HCO.sub.3.sup.- .fwdarw.UO.sub.2 (CO.sub.3).sub.3.sup.-4 +H.sup.+ PA1 MoS.sub.2 (S)+9[O]+3H.sub.2 O.fwdarw.MoO.sub.4.sup.-2 +6H.sup.+ +2SO.sub.4.sup.=
When oxygen dissolved in the lixiviant is used as the oxidant, the reaction essentially follows the overall reaction applicable to any oxidation of tetravalent to hexavalent uranium:
This hexavalent uranium is dissolved in the lixiviant by the formation of a soluble uranyl carbonate complex:
The overall CO.sub.2 /O.sub.2 leach reaction, therefore, may be given as:
This leaching process has the obvious advantage of avoiding the use of ammonium ions, as in the ammonium carbonate/bicarbonate process, wherein ammonium ions may be exchanged onto the sodium and calcium smectite clays found in the South Texas uranium-bearing formations, creating a possible threat of groundwater contamination. This CO.sub.2 /O.sub.2 leaching system works well in formations wherein oxidant consumption is moderate.
However, it has been found that many uranium formations contain large amounts of reducing compounds, such as H.sub.2 S and other sulfides, hydrocarbon gases and other organic matter, which act as oxygen scavengers. For example, many of the roll-type formations which are notably suitable for in situ uranium leaching contain MoS.sub.2 and FeS.sub.2 as well. These compounds, as well as the other sulfides and organic compounds referred to above, preferentially consume the oxygen available in the injected lixiviant, effectively inhibiting the solubilizing of uranium until most or all of these scavengers are oxidized. The side reaction with molybdenite, MoS.sub.2, poses yellowcake contamination problems as well as producing acid as it consumes oxidants:
In many formations this scavenging or reducing capacity is so high that the leaching rate is limited by the supply of oxidants. This is particularly true where the CO.sub.2 /O.sub.2 leaching system is used because the solubility of O.sub.2 in the leaching solution is low; the scavenging of the O.sub.2 supply is most marked at early stages of the leaching operation. In the typical in situ leaching operation, where the lixiviant, or leach solution, is injected into one well and the pregnant lixiviant, or leachate, is produced from other wells spaced at a distance, no uranium will be produced in the pregnant lixiviant until the entire formation is essentially oxidized.